In general, where a drug is administered in the body, there is used a drug administration device (syringe) including an outer tube and a plunger. The plunger is so operated that the drug filled in a fluid chamber of the outer tube is discharged into the body through an injection needle connected to the tip of the outer tube.
The skin is made up of three layers including epidermis, dermis and subcutis. The epidermis is a layer having a thickness of about 50 to 200 μm from the skin surface and the dermis is an about 1.5 to 3.5 mm thick layer lying adjacent from the epidermis. Since an influenza vaccine is generally subjected to subcutaneous or intramuscular administration, it is administered in the lower region of the skin or a deeper section thereof.
On the other hand, in order to reduce an injected quantity of the vaccine, it has been investigated to administer the influenza vaccine to the skin upper section as a target site, in which immunocompetent cells are abundant. It will be noted that the skin upper section indicates the epidermis and dermis of the skin.
The skin upper section contains abundant elastic fiber tissues such as elastin, collagen and the like and is harder than the subcutis. Accordingly, the administration into the skin upper section needs a higher injection pressure than the administration into the subcutis because a high back pressure is caused to generate at the skin upper section upon drug injection. It is to be noted that a back pressure generating upon drug injection into the skin upper section according to the Mantoux method using an ordinary syringe is at about 0.8 to 1.5 MPa.
A technique of administering a drug in the body may be one as described in Patent Document 1. The syringe described in Patent Document 1 includes a capsule having a drug solution filled therein, a syringe body, and a piston rod. The piston rod is operated so as to permit the drug solution filled in the capsule to be injected into the body via an injection needle of the syringe body.
The technique of increasing an injection pressure upon drug administration in the body may be one as described in Patent Document 2. The pressure syringe described in Patent Document 2 includes a nozzle having a drug solution chamber, a piston (piston rod), and a release button. The piston is moved at high speed by operation of the release button to generate a high pressure, resulting in the injection, into the body, of the drug solution filled in the drug solution chamber via an injection needle provided at the nozzle.